Two independent cell lines established from m1MSV (BMuX) induced fibrosarcomas in Japanese quail showed changes in the size and cleavage stability of the m1MSV specific pP60gag polyprotein. The size of the gag polyprotein in these cell lines was 2000 and 6000 daltons less than pP60gag. The change in molecular size of the new polyproteins was not altered in infected rat cells which indicated genetic alterations in the m1MSV genomes. A phosphokinase was isolated from MSV (MMuLV) pseudotype virions by actin affinity chromatography that interacted with antibodies to pP60gag. This demonstrated an immunological interaction between a sarcoma specific polypeptide and a cytoskeletal polypeptide.